This invention relates to a process for producing a multi-layer printed wiring board having high reliability with high efficiency by an additive process and a composite film used therein.
Multi-layer printed wiring boards applied to various electric products are generally produced by laminating a plurality of inner circuit substrates and outer circuit plates via prepregs and binding them (e.g. Japanese Patent Examined Publication No. 38-14977). Under such circumstances, since a large apparatus and long steps are necessary, there is a limit to lowering the production cost. Further, since unevenness of inner circuit substrates appear on surfaces of outer layers, defects are readily formed at the time of forming out circuits. This causes a limit to a wiring density. In order to improve this, there is proposed a process comprising forming an insulating resin layer on an inner circuit substrate, forming a resin adhesive layer for electroless plating, curing these resins with heating, and forming outer layer circuits and through-hole conductors by electroless plating using an additive process (e.g. Japanese Patent Unexamined Publication No. 60-5198895).
A typical additive process is explained referring to FIGS. 6(A) to 6(C). On both sides of a printed circuit substrate 3 obtained by forming copper patterns 2 on both sides of an insulating substrate 1 (FIG. 6(A)), insulating layers 4 are formed (FIG. 6(B)), followed by forming additive layers 5 thereon (FIG. 6(C)). Surfaces of the additive layers 5 are roughened (surface treatment for imparting affinity for water) j with a chromic acid mixed solution (CrO.sub.3 +H.sub.2 SO.sub.4), and subjected to a surface activation treatment for providing a catalyst such as palladium chloride on the surfaces, provided that no catalyst is adhered previously. The surfaces are masked with a photosensitive lacquer (a photo resist) by a photographic method or by a screen printing method (formation of resist films) on non-circuit forming portions, followed by electroless plating on circuit forming portions. Lamination of the insulating layers 4, followed by lamination of the additive layers 5 are conducted by processes (under reduced pressure) as disclosed in Japanese Patent Unexamined Publication Nos. 62-236726 and 62-236727. As the printed circuit substrate 3 shown in FIG. 6(A), there are generally used copper-clad laminates formed by a conventional substrate process. But the process as shown by FIGS. 6(A) to 6(C) are complicated and requires much more time and much more labor.
With recent requirement for miniaturization and higher performance of electronic devices, multilayer printed wiring boards are required to have a higher density. In order to have a higher density, the width of conductors is required to be smaller, which results in requiring smoother surfaces. In order to meet such a requirement, Japanese Patent Unexamined Publication No. 51-60958 proposes to laminate a transferring sheet obtained by previously coating a resin on a base plate on a printed wiring substrate in place of coating a resin thereon. By this method, uniform thickness of the resin layers can be obtained and surface smoothness can be obtained to some extent, but there are still problems in that adhesiveness between the resin and conductors is insufficient, difficulty in practical application is caused by narrowing the circuit conductor width in order to obtain the higher density.
On the other hand, in order to quickly remove heat generated by parts mounted on printed circuit boards, to have high thermal conductivity is required. For such a purpose, a metal plate having good heat dissipation is used as a substrate for printed wiring boards. Such a metal plate has a number of through-holes with insulating layers formed on surfaces thereof. The insulating layers are formed by electrodeposition of an insulating varnish, powder coating of an insulating resin composition, and the like. According to the electrodeposition method, coating film with uniform thickness can be obtained but there are problems in controlling of an electrodeposition solution, disposal of waste liquor, and the like. On the other hand, according to the powder coating method, there are problems in that the thickness of a resin layer at through-hole edge portions which are boundaries of through-holes and plane portions is remarkably reduced by agglomeration of molten material due to surface tension at the stage of melting and curing the powder on the metal plate, pin holes are easily produced at the edge portions concentratively due to concentration of shrinking stress at curing caused by the difference in thickness. The presence of pin holes is a fatal defect in function of printed wiring boards.